Well suddenly, implementation day is upon us. This is the day that Iran will, in theory, complete everything it needs to do implement the nuclear deal it reached with world powers last July. And for Iran, this means the liftings of sanctions that have done a lot of damage to its economy. Let's turn to my colleague, who's been covering the story for - probably, it seems a very long time to him. NPR's Peter Kenyon, who joins us from Istanbul. Good morning, Peter.

PETER KENYON, BYLINE: Hi, David.

GREENE: So one of the major steps that Iran agreed to hear to cut back its nuclear program involved the reactor it was building at Arak, which can produce plutonium for an actual atomic weapon. What is the latest there?

KENYON: Well, we've had some conflicting statements on that coming out of Tehran. But Secretary of State John Kerry says he's heard directly from Iranian Foreign Minister Mohammad Javad Zarif that that reactor core has been pulled out. It'll be filled with cement, and that should just about eliminate the threat of a plutonium-fueled nuclear weapon. And that's because of something else that Kerry talked about, something that really hasn't got that much attention - the new inspection and monitoring powers to be had by the IAEA, the International Atomic Energy Agency. And here's a little bit of what Kerry says about that.

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JOHN KERRY: In the meantime, the IAEA will build up its capacity to inspect and know what Iran is doing and for 25 years, will be tracking every bit of uranium that is processed, from the mine, to mill, to the gas, to the yellow cake, to the centrifuge and into the waste.

KENYON: Now he got a little technical, but basically what he's saying is that the U.N. inspectors should be able to track every gram of uranium, the nuclear fuel, from the minute it's mined all the way through the enrichment process. The idea being that if they can track all the fuel from start to finish, even if there was an attempt to launch a covert Iranian nuclear weapons program, there wouldn't be any fuel for it.

GREENE: I suppose that long list there from Kerry, in a way responding to critics who have long said, you know, how are you actually going to make sure that Iran fulfills its end of the deal? Kerry basically is saying we've got it all covered.

KENYON: He is, and the critics are not satisfied with that explanation, of course, so we're going to hear this debate go on for some time.

GREENE: Well, for Iran, does implementation day actually mean an end of sanctions, or an end of some sanctions at least, and this massive influx of cash that they've been waiting for?

KENYON: It does. The EU, the U.S., others say they're ready to hold up their end of the bargain. Details are in short supply at the moment, but there are possibly 100 billion or more dollars in assets that are waiting to be unfrozen, and then officials in Tehran are really focused on these banking sanctions. All kinds of transactions have been blocked, and that's hit all sectors of the economy. On the other hand - critics, meanwhile, are going to be saying wait a minute. All this money and this reinvigorated Iran is going to make things much more volatile when we've already got these wars going in Iraq, Syria and Yemen.

GREENE: Wars going, and also - I mean, all the news recently of the tensions between Iran and Saudi Arabia. Given all of this turmoil in the region, is it significant that this deal actually appears that it might be implemented and go through?

KENYON: I think it is pretty significant. I mean, you can look at the spike in tensions between Iran and Saudi Arabia, perhaps, as Riyadh's effort to see if this might be derailed. It hasn't happened. There's going to be years of suspicion still to come. But take another example, the U.S. sailors detained in the Persian Gulf this week. They were freed in less than 24 hours. Would that have happened without the countless hours Kerry and Zarif had spent together not only talking, but making commitments that both countries have so far kept? I mean, that's a pretty important change.

GREENE: All right, NPR's Peter Kenyon speaking to us from Istanbul. Peter, thanks.